In a great many industrial fields, weight reduction of parts is becoming of increasing importance. The automotive industry is an excellent example of this trend. In the current pressure to conserve energy, auto manufacturers are designing cars that will conserve fuel, and a large part of this effort is being accomplished by reduction in weight of all possible parts going into the making of an automobile. Aluminum and plastics are being substituted for steel in applications where this is appropriate. Similarly, many attempts are being made to substitute plastic foams for prior conventional fabrics and/or solid plastic materials. One of the problems encountered in the use of foams, especially low density, lightweight foam materials, is that articles made therefrom must still be fastened to other parts, or secured in place in an intended assembled position, and foam articles require reinforcement or other means to be useful in such an application. The loss in strength encountered when a material is changed into a light low density article made of foam can defeat its utility and usefulness. Accordingly, the making of foam articles, having reinforced areas or regions, such as regions of higher density and strength, is of tremendous advantage in promoting the use of foam articles for lightening the weight of vehicles.
It has been proposed to incorporate supplemental and stronger fastening elements within the foam article to overcome this problem, but if the foam is of insufficient strength, then it may not have the holding power to securely hold the supplementary fastening means, and this does not provide a satisfactory solution. If the lightness of the article results in a sufficient loss of strength, then there can be no assurance that the article is capable of holding any such supplementary fastening means.
The solution provided by the present invention is to provide a lightweight, low density foam article, but which can be provided with higher density, greater strength regions or portions for use, for example, for fastening.
By way of illustration, a foamed styrenic copolymer thermoplastic composition having a density of 5 to 6 pounds per cubic foot, will have an average screw-pull strength of only about 50 pounds or less. The same material, if it had a density of say about 12 to 13 pounds per cubic foot, would have an average screw-pull strength of about 150 pounds; and at 15 pounds per cubic foot density, the average screw-pull strength of the foam material would be about 200 pounds. Thus, the present invention has for a specific object thereof the provision of a process and apparatus for making foamed articles, having generally a relatively low density overall, but which in the particular desired regions or portions will have a higher density of, say 12, 15, or even 20 pounds, and thus provide desired strength in these particular areas or regions.